by B.B. Pelletier

Part 1

We get requests all the time for basic maintenance articles and fundamental articles about how to diagnose an airgun and make it shoot better. Often, I refer readers to blog reports I’ve done in the past, but today (and tomorrow!) is a blog report with something for almost everyone. It started out as a simple test of my Air Venturi Bronco rifle with different sights, but it blossomed in several different directions — answering many questions and raising issues about which many readers have indicated an interest in the past.

I didn’t plan on this report turning out the way it did. This special two-part report (today and tomorrow) is a serendipitous journey of airgun discovery. It began when I mounted the optional Bronco Target Sight kit on my rifle and thought I would be demonstrating just how accurate a Bronco can be. (The Bronco is also sold with the target sight kit installed.)

Heck (I thought), there’s no risk here. I’ve shot this same Bronco before and I know how accurate it is. What could possibly go wrong?

What, indeed. If you click on the link to Part 1 of this report provided above, you’ll see that the first groups I shot with the Bronco and its new target sights were anything but encouraging. If this had been a different rifle I might have been tempted to write it off as inaccurate (I said tempted — not a sure thing), but because I’ve shot this exact same Bronco several times in the past with great results, I knew it was something other than the rifle’s inherent accuracy at fault.

Back-bored muzzle
You readers guessed what the problem could be, and Mac and I conversed at the same time. Mac owns a Bronco, too, and he knows how accurate it is.

One early theory was that the new longer front sight mounting screws might be protruding into the rifled barrel and clipping the pellet just before it leaves the muzzle. I had noticed when mounting the four front sight riser plates that the screw holes are drilled deep, so I checked them and discovered they are drilled all the way through the barrel. The kit has two longer screws that are needed because of the four riser plates, so was it possible that one of those screws was hitting the pellet as it passed through the bore?


The four front sight riser plates require longer mounting screws. Was one of them touching a pellet?

But Mac told me the barrel was back-bored. The muzzle is not where you think it is, but it’s about seven inches deep inside the barrel — behind the front sight. Back-bored means that instead of being crowned at the end of the barrel, the muzzle is sunk deep inside the barrel with a deep-hole drill. Doing this protects the muzzle from damage and preserves accuracy longer. It can also restore accuracy to rifles that have been cleaned from the front instead of the breech. When cleaning rods scrape against the sides of the muzzle they wear the metal and cause accuracy loss. Mosin Nagant rifles are often found with back-bored muzzles.


The true muzzle of the barrel is located at the tip of the cleaning rod. So, the front sight screws are not interfering with pellets before they leave the barrel.

Look inside the barrel
Once I confirmed where the true muzzle was, it was obvious the front sight screws could not be interfering with the pellets before they left the muzzle. But what about after they left? Could a pellet still be touching the edge of a screw after it exited the muzzle?

This time, the answer was less exact. From what I could see with an endoscopic light down the bore, the screws were probably not protruding deeply enough to touch the pellets in flight unless the pellets were yawing wildly. And if they were yawing wildly, they were never going to be accurate anyway. So I stopped looking at that and checked the cleanliness of the barrel next.

Surprise!
The barrel had a constriction right at the muzzle! A brass bore brush passed from breech to muzzle (the true muzzle — not the end of the barrel) stopped abruptly at the muzzle. Something was constricting the barrel right at the point the pellet exited. The barrel needed to be cleaned, but this constriction was so abrupt that it felt like a large burr had been raised right at the muzzle. But the muzzle is seven inches deep inside the barrel, so that’s next to impossible!

The only solution was to clean the barrel, and I started with a clean brass bore brush. Don’t waste your time with a nylon brush. It isn’t stiff enough to remove the metal if there’s any. As long as the barrel is made of steel, like the Bronco’s barrel is, you cannot damage it with a brass brush.

After 20-30 passes the brush was meeting no resistance, so I then cleaned the barrel with Otis bore solvent until the patches came out clean. Now, the barrel was ready to perform at its best!

On to shooting
After all of this, I felt ready to shoot the rifle and expected it to do well. A quick read of Part 1 showed me that I tested it with H&N Finale Match Rifle pellets and JSB Exact RS pellets. Those were the first two pellets I shot. The range was 10 meters, and each group got 10 shots.

I would have loved to have shot two beautiful targets and ended this test right there, but that didn’t happen. Yes, the groups were somewhat smaller than those shot in part 1, but they were not good groups — not for a Bronco, anyway. The H&N Finale Match Rifle pellets grouped 10 in 1.059 inches — compared to the group in Part 1 of 1.668 inches. And the JSB Exact RS pellets grouped 10 into 0.82 inches this time –compared to 1.169 inches in Part 1. Yes, these are smaller groups, but they aren’t small enough for the Bronco at 10 meters. Something else had to happen.

I found the secret
This is where I’m going to end the report today. Tomorrow, I’m going to tell you what happened to change the outcome of the test. Yes, a different pellet was used, but that wasn’t the big news. In fact I’m convinced that I’ve found the secret to increasing accuracy with any Air Venturi Bronco — shooting any pellet!

You have a day to wonder, ponder, guess and discuss. Let’s see how smart you guys are.

What did I do that was different? A hint — I’ve done it before with similar results.